1. Field of the Invention
This invention relates to a ceramic honeycomb structural body having a plurality of open-ended cells defined by cell walls.
2. Description of Related Art
A filter comprised of a ceramic honeycomb structural body has previously been used as a catalyst substrate or a fine particle filter for purifying an exhaust gas of an internal combustion engine, or a filter used for purifying or deodorizing a combustion gas from petroleum or various gases as a fuel. FIG. 4 is a sectional view perpendicular to open-ended cells illustrating an embodiment of the conventional ceramic honeycomb structural body. In the embodiment of FIG. 4, the ceramic honeycomb structural body 51 of a columnar shape made from a material such as cordierite or the like comprises a plurality of open-ended cells 53 defined by cell walls 52 and extending parallel to each other. Part of the open-ended cells 53 is shown enlarged in FIG. 5, open-ended cells 53 in the conventional ceramic honeycomb structural body 51 have, in many cases, a square shape on a surface perpendicular to the longitudinal direction of the open-ended cells 53, while each of corner 53a has a right-angled shape crossing surfaces of the cell walls 52 to each other.
In the ceramic honeycomb structural body 51 particularly used as a catalyst substrate for the purifying the exhaust gas of the internal combustion engine, it is recently required to have thinner partition cell walls 52 and a greater number of open-ended cells 53 so as to satisfy conditions such as an increase of purification efficiency of the exhaust gas, a reduction of pressure loss, use at a higher operating temperature and the like. The ceramic honeycomb structural bodies having thinner cell walls 52 in accordance with the aforementioned requirement have such problems that the structural body is liable to collapse due to thinner cell wall 52 and the mechanical strength is low. Also, there are problems that it is very difficult to manufacture a jig (die) required for the production of the structural body and the number of the manufacturing steps is numerous. Further, is difficult to obtain a jig at a high working accuracy and the cost and delivery time are increased, and the like. Furthermore, even in the forming of the structural body, it is important to prevent the collapse and deformation at the forming step.
In order to improve the mechanical strength of the ceramic honeycomb structural body 51, there have hitherto been known techniques disclosed in JP-A-54-110189 and JP-A-54-150406. According to these techniques, the thickness of the cell wall located at an outer peripheral side of the ceramic honeycomb structural body 51 is made thicker than that located at a central side thereof, but there are caused problems that the thickening of the cell wall 52 narrows an effective open-ended cell 53 and increases the weight of the structural body to lower the thermal shock resistance.
Further, as a technique relating to four corner parts 53a of each open-ended cell 53 in the ceramic honeycomb structural body 51, there have been known techniques disclosed in JP-A-49-113789 and JP-A-56-147637. According to these techniques, a round or straight swollen portion or fillet portion is disposed on each corner part 53a of all open-ended cells 53 in the ceramic honeycomb structural body 51. In this case, however, there is a problem that it is very difficult to manufacture a forming jig used for producing the ceramic honeycomb structural body 51 having thinner cell walls 52. This problem becomes conspicuous when the thickness of the cell wall is thinner (not more than 0.15 mm). Furthermore, there is caused a problem that a rate of lowering the mechanical strength of the structural body or causing the collapse in the forming is increased.